The present nonprovisional application claims priority under 35 USC 119 to Japanese Patent Application No. 2002-131938 filed on May 7, 2002 the entire contents thereof is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an automatic transmission in a power unit mounted on a four-wheel buggy (saddle type vehicle for rough terrain running) or the like, wherein the power unit is configured by integrating an internal combustion engine with a torque converter and a transmission having a hydraulic clutch.
2. Description of Background Art
In a transmission having a hydraulic clutch adapted to be engaged and disengaged for the switching of speed-change gears, a friction loss is reduced by increasing a contact pressure hydraulically applied to a clutch plate. Although the contact pressure can be increased by increasing a clutch capacity, the clutch is undesirably increased in size. Increasing the contact pressure is required at the time the vehicle is accelerated to operate the clutch. A conventional power unit has no means for increasing an hydraulic pressure supplied by an oil pump in operating the clutch.
It is accordingly an object of the present invention to provide an automatic transmission having a function of increasing a hydraulic pressure applied to a clutch only when accelerating a vehicle to operate the clutch.
According to the present invention, an automatic transmission in a power unit with a torque converter and a hydraulic clutch is provided that are adapted to be engaged and disengaged for the switching of speed-change gears. The automatic transmission includes a pressure control valve provided at a discharge port of an oil pump for supplying a hydraulic oil to said hydraulic clutch. The pressure control valve includes a valve body, a valve element axially slidably provided in said valve body, and a spring provided between said valve element and said valve body in a sliding direction of said valve element for adjusting a set pressure for opening/closing of the pressure control valve. The valve element is urged in its closing direction by a repulsive force applied to a stator in the torque converter.
With this configuration, at acceleration of a vehicle having the automatic transmission according to the present invention, a large hydraulic pressure is supplied to the hydraulic clutch, so that a power transmitting rate can be increased without the need for increasing the size of the clutch.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.